A prior art internal combustion reciprocating engine typically includes an air intake manifold that receives air through an inlet and distributes the air through a plurality of outlets and runners to respective engine cylinder intake ports. Camshaft-operated intake valves open to allow the flow of air and other combustion gases, such as fuel vapor or recirculated exhaust gas, through the intake ports. The geometry of the intake ports is typically configured to cause the air to swirl inside the cylinders.
The desirability of generating swirling motion to an induction charge about the axis of an engine cylinder within a combustion chamber is well known. Such swirling motion produces a number of advantages in both the charge preparation within the combustion chamber and in the combustion of this charge within the combustion chamber. For example, in a conventional spark-ignition homogeneous engine operation, increasing swirl rate (usually defined as the ratio of in-cylinder charge rotative speed to engine rotative speed) has generally increased the burning rate and resulted in decreased fuel consumption. In stratified charge engines, some amount of swirl is also employed to promote mixing between the rich core of fuel and the surrounding air in order to reduce exhaust emissions and fuel consumption. High swirl rates may also be beneficial for maintaining a centrally located fuel-air cloud in certain types of stratified charge engines. Swirl is also used in diesel engines to promote fuel-air mixing for lower NOx emissions and soot formation.
The amount of swirl in a cylinder, as measured by in-cylinder angular momentum flux, is determined in part by the amount of mass flow through an intake port and the ability of the port to generate swirl. Variations in the mass flow rate schedule among a plurality of cylinders can cause substantial differences in the amount swirl generated among the cylinders. The mass flow rate through an intake port is directly related to the difference in pressure upstream of the port/runner and downstream in the cylinder. Thus, any pressure fluctuation differences in the manifold during the intake process for the cylinders can cause inconsistent swirl among the cylinders.